1. Filed of the Invention
This invention relates to the use of certain water-soluble polyether polyols as additives for aqueous coating compositions wherein the binder is based on a combination of water-dilutable polyacrylate resins and organic polyisocyanates.
2. Description of the Prior Art
Ecological considerations play an important role in the coatings technology. One particularly urgent problem is the reduction in the quantities of organic solvents used for lacquers and coating materials.
For a long time it was to possible to dispense with organic solvents for chemically cross-linking polyurethane lacquers which play an important role in coatings technology due to their excellent properties. It appeared for a long time that it would not be possible to use water instead of organic solvents in two-component polyurethane lacquers based on polyisocyanates containing free isocyanate groups since it was known that isocyanate groups reacted not only with alcoholic hydroxyl groups but also with water. Moreover, the concentration of active hydrogen atoms in such systems due to the presence of water is much higher than the concentration of hydroxyl groups from the organic, isocyanate reactive component. It was therefore inevitably assumed that the main reaction in the ternary system of polyisocyanate/organic polyhydroxyl compounds/water would be a reaction between isocyanate and water with the formation of urea and carbon dioxide. This would result in the organic polyhydroxyl compounds remaining unreacted and would cause foaming of the reaction mixture due to the formation of carbon dioxide.
Aqueous two-component binders based on water-dilutable polyacrylate resins and organic polyisocyantes were first described in EP-A-O 358 979. The coating compositions described in this publication result in films which have excellent hardness after curing but insufficient gloss for many purposes. Further, the elasticity and abrasion resistance are insufficient for many applications. These properties are required, for example, for coating wood and furniture and for coating flexible substrates such as cork or synthetic resins.
It has now surprisingly been found that the addition of small quantities of water-soluble polyether polyols of the type described below to such aqueous coating compositions results in a marked improvement in the gloss, elasticity and abrasion resistance of coatings obtained from the coating compositions.